Conventional electrical switches use a moving contact to change from the "off" to "on" state or from a normally open to a normally closed position. Withstanding moderate currents makes these contacts relatively large; and tolerating high voltages, for example, 250 volts, between the contacts increases contact separation. This property makes the fabrication of small switches difficult and, in some cases, reduces the longevity of the switch contacts.
Electrical switches are used in many applications. However, they have problems when applied to explosive environments because spark hazards which exist from contact opening and closing can cause ignition of the surrounding atmosphere. In these situations, the switches are typically sealed to prevent explosion problems. This increases the cost of the switches by orders of magnitude.
Other situations where electrical switches are not welcome involve applications where the wires which connect to the switches become a system liability. Two examples of this sort are where, in military systems, electromagnetic pulses from nuclear explosions can generate spurious signals or even damage electrical circuits. Another problem area is in the unwanted radiation of signals. This is a particular concern in equipment which is used for the transmission or encryption of classified information. When conventional switches are used in this application, the opposite process takes place. That is, radiation from the wires connected to the switches may be detected, thereby allowing disclosure of the classified information.
Replacing these wires with optical fibers is attractive in the situations described above. It is possible to mimic electrical switches, for example, by selectively altering the position of a mirror. Thus, switch action may rotate a mirror, allowing light coming from a light source along a fiber to be directed to either one of two other optical fibers, each communicating with a respective photodetector. Such systems, although simple in concept, are difficult to fabricate and involve several disadvantages. First, the fact that three fibers are required increases system cost if the distance traveled is more than a few meters or if complex connectors are required between the switch and electronics. Generally, alignment of a scheme of this sort is a problem because the mirror must be positioned within a very tight tolerance in order to couple light back into the fiber.
A single-fiber version of a movable mirror optical switch can be implemented by selectively moving a mirror in a plane between two positions, one of which is directly in front of the outlet end of an optical fiber. When the mirror is positioned in front of the outlet end of the fiber, light from the light source is reflected by the mirror back along the same fiber toward a photodetector at the other end. Although this single-fiber system is simpler than the above-described two-fiber system, it suffers from another problem which is common to all duplex fiber links. That is, light backscattered from the source end of the fiber gets into the detector. Also, Rayleigh backscattering and back reflection from connectors along the system reduce the available signal to noise in the detector circuit. Variations in the amount of the losses, backscattering and back reflection can result in ambiguities in the intensity of the detected light. In other words, for a given arrangement, the detected light for optimal conditions where the mirror is not positioned in front of the fiber can be greater than when the mirror is positioned in front of the fiber, but losses, backscattering and back reflection are less than optimal. Additionally, single-fiber systems suffer from a problem if connectors are present because, as a connector removed and replaced, it may have completely different light transmission and reflection characteristics, causing the system to not be able to differentiate between the presence or absence of the mirror at the fiber end.